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Comments - COUNT DISTINCT

1 year, 10 months ago Archie Cobbs
Re: COUNT DISTINCT

Question: Is COUNT DISTINCT optimized to take advantage of sorted values?

For example, if I have a sorted column LAST_NAME, then SELECT COUNT(DISTINCT(LAST_NAME)) FROM PEOPLE can be performed in a constant amount of memory: if you iterate the rows in LAST_NAME order, then you're guaranteed that all distinct last name values appear contiguously, so there's no need to remember which one's you've already seen.

 
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